Fiber-reinforced composite materials with thermosetting resins including epoxy resins as a matrix are known. For example, Japan Unexamined Patent Publication No. 2011-99094 discloses an epoxy resin composition containing an epoxy resin as a matrix, a thermoplastic resin for viscosity adjustment, a filler, and a curing agent, and a prepreg obtained by combining the composition and a reinforcing fiber. Such a prepreg is used in a wide range of fields including structural materials such as aircrafts and vehicles, reinforcing of concrete structures, sports fields such as golf clubs, tennis rackets, and fishing rods because of its lightness and excellent mechanical properties.
One of the characteristics required for the prepreg used in such applications includes heat resistance. Therefore, various fiber-reinforced composite materials having a resin with higher heat resistance than epoxy resins as a matrix have been investigated.
Resins that may substitute for epoxy resins include cyanate ester resins. Cyanate ester resins have superior heat resistance than epoxy resins, and can withstand heat up to near 300° C., for example.
However, cyanate ester resins have the problem that the thermoplastic resin for viscosity adjustment is difficult to dissolve, the viscosity adjustment of the resin composition is difficult, and the resin has a high viscosity at room temperature but has a low viscosity during curing heating. Due to this problem, when the prepreg is heat cured, the resin composition flows out of the reinforcing fiber, and resin defects occur in the obtained fiber-reinforced composite material, and the thickness is non-even. In order to suppress resin flow during curing, the viscosity of the resin composition needs to be higher, but in this case, the viscosity of the resin composition at room temperature is too high, so the workability during prepreg molding will be negatively affected.